This invention relates to electric storage batteries and more particularly to storage batteries having enhanded operating characteristics.
The battery industry has seen increased demand for battery management technology, primarily due to the consumers"" ever-increasing appetite for the convenience of battery-powered portable equipment such as cellular phones and laptop computers. Additionally, the battery industry is seeing a movement toward an increased emphasis on electric motor-driven tools and zero emission vehicles with the primary power source for these new generation vehicles being batteries. This movement is due to rapidly increasing government regulations and consumer concerns about air and noise pollution. Another area which requires high efficiency batteries is energy storage applications such as load-levelling, emergency/standby power and power quality systems for sensitive electronic components.
As a result of the increasing demand of battery-powered equipment, the battery industry is under competitive pressure to produce an ideal cell. A cell that weighs almost nothing, takes up no space, provides excellent cycle life and has ideal charge/discharge performance and does not itself produce an environmental hazard at the end of its life. The most popular technology utilised by the battery industry is the lead-acid battery, which is being challenged to meet higher energy density, smaller size, better performance levels, longer cycle life and guaranteed recyclability.
Conventional lead-acid batteries suffer from limited capacity utilisation, low depth of discharge, short cycle life, low energy density, thermal management problems and the need for constant boost charging to maintain cell equalisation.
The lead-acid batteries also require long charge times and high charge currents can only be used for a few minutes at very low states-of-charge. If high currents are used it normally results in higher than allowable voltages being reached leading to electrolyte loss and a reduction in the battery""s capacity. The time to recharge a lead-acid battery with boost charging can be up to 4 hours at best if a proper charge profile is followed.
The cycle life of lead-acid batteries varies greatly depending on the Depth-of-Discharge (DOD) reached during cycling. For electric vehicle applications a 90-100% DOD may not be uncommon and at these DOD levels the cycle life of conventional deep cycle lead-acid batteries would be approximately 300 cycles. As most controllers function on the total battery voltage it is not uncommon for individual cells to be discharged below an acceptable limit as the overall battery voltage technique relies on the assumption that all cells are at the same state-of-charge, which is usually not the case in practise. Systems can be so far out of balance that under high loads individual cells can actually reverse and even gas during the discharge. This may seem extreme, however, when a large battery array is used to provide power at higher voltages cell reversal may occur without being detected initially.
Conventional NiMH batteries employ advanced processed and high purity materials. This leads to a very high cost for the battery systems. Expanded nickel foams with high purity nickel hydroxide compounds and processed metal alloy materials all need a very high degree of quality control in order to obtain a high performance battery.
NiMH hydride batteries can also suffer from self-discharge problems and can also be affected by temperature. On certain systems the extraction of high current can cause damage to the battery cells and care must be taken not to over charge the batteries. In this respect, advanced battery chargers are needed to ensure proper charging.
Redox batteries have been under investigation for may years and have mainly been in the form of flow batteries. Redox flow batteries store energy in the liquid electrolytes which are stored separately to the battery stack. During operation the electrolytes are re-circulated through the system and energy is transferred to and from the electrolytes. When charging, electricity is transferred to and stored by the electrolytes, upon discharge, the electrolyte release the stored energy to the load. Redox flow batteries typically have a low energy density and incur pumping losses associated with re-circulating the electrolyte through the system. In certain cases, high self-discharge rates are possible depending on the membranes or the existence of internal leaks and shunt currents.
According to one aspect of the invention there is provided a redox gel battery comprising at least one cell consisting of a positive redox gel electrolyte, a negative redox gel electrolyte, a membrane between the positive and negative redox gel electrolytes, a positive electrode electrically connected to the positive redox gel and a negative electrode electrically connected to the negative redox gel electrolyte.